The subject matter disclosed herein relates generally to driver circuits for a load, and, more particularly, to a driver circuit that will allow a load, such as an LED, to be operated over a wide input voltage range.
A variety of different electrical power sources can be used to power an LED driver circuit. These include both alternating current (AC) and direct current (DC) power sources, such as those provided by wall sockets (120 or 240 VAC at 50 or 60 Hz) or DC power supplies (typically 24 VDC to about 240 VDC) and the like. Conventional AC power sources, such as those provided by wall sockets from power lines (line voltage) are typically 110 or 220 VAC at 50 or 60 Hz, and other voltage and frequency combinations are used throughout the world. AC voltage sources must first be rectified to become DC voltage sources before using them to drive LEDs. Further, line voltage components that would be required to compensate for these problems are usually rather large and more expensive than their lower voltage counterparts.
Use of a rectified AC supply voltage, or a pure DC supply voltage, avoids some of the complications of an AC input, but is not without complications of its own. For example, the actual output voltage of any given voltage source is rarely exactly equal to its specified nominal voltage. As an example, the nominal output voltage of 12 VDC battery systems like those used in automobiles is seldom exactly 12 VDC, but rather typically varies between about 11 VDC and about 15 VDC.
One difficulty associated with LED driver circuits in general is the large number of catalog numbers that need to be manufactured and warehoused. Typically, an LED driver circuit is designed for only one specific supply voltage. If you are a manufacturer, you want to offer a full product line, which means offering a large variety of driver circuits that operate at their respective supply voltage. If you are an integrator or an OEM using LEDs, this mean that you need to have available a large selection of driver circuits that operate at different voltages for your application's needs. Attempts to accommodate LED driver circuits to operate on more than one supply voltage results in increased size, cost, and heat generation.
Supply power disruptions such as voltage dips and interruptions are common to industrial control circuits and can produce undesirable effects such as LED flickering or dimming. Ride-through is a term used to describe the ability to withstand voltage dips and interruptions with steady LED illumination. During a voltage dip for a conventional design, a low voltage drop would occur across a current regulation resistor, which would cause the transistor to switch off and the LED to dim or turn off.
Still other difficulties associated with LED driver circuits reside in the presence of leakage current from upstream circuitry used to energize the LEDs. Even low levels of leakage current can cause an LED to slightly illuminate even when the driver circuit is in an off-state.
There is a need, therefore, for an improved driver circuit that will allow a load, such as an LED, to be operated over a wide input voltage range, while at the same time, that can improve both ride-through and surge capabilities, and require fewer catalog numbers.